A semiconductor wafer used for producing a semiconductor device, is produced by, for example, slicing a silicon single crystal ingot grown by a Czochralski method and thereby to process the ingot into a wafer shape, and then going through the respective steps of chamfering (grinding), lapping, etching, single-side polishing, chamfered-portion polishing (mirror chamfering), and so forth.
In recent years, along with rationalization of a process for producing a semiconductor device and reduction of cost, improvement of yield of device chips per one wafer has been required. In the case of silicon wafer, as the measures for improving the yield of device chips, large-diameter wafers have been more used, and a so-called peripheral excluded region of the wafer from which device chips are not obtained has been scaled-down.
For obtaining square-shaped chips from a circular wafer, a wafer having a large diameter has become advantageous. As well as in DRAM that has been conventionally a main stream, in production of recent flash memories for digital consumer electronics, wafers having a diameter of 300 mm is used and its production amount has drastically increased.
Moreover, the peripheral excluded region of the wafer has been scaled down from conventional 3 mm to 2 mm so that chips can be obtained from a larger range of the wafer. Furthermore, requirement of a peripheral excluded region of 1 mm has come out.
In the process for producing a silicon wafer having a large diameter of 300 mm, which is different from a method for producing a wafer having a diameter of 200 mm or less, a double-side polishing step of polishing front and back surfaces at the same time has been generally adopted for obtaining higher-precise flatness quality or nanotopographic quality in the wafer. In this case, for example, as shown in FIG. 15, a mirror-like silicon wafer can be obtained by going through the respective steps of slicing, chamfering, lapping (double-disc grinding, surface grinding), etching, double-side polishing, mirror chamfering, final polishing, and so forth.
The double-side polishing step is performed by using such an apparatus 70 as shown in FIG. 4. In the apparatus 70, wafers W are contained in circular holes 78 of carriers 75 as shown in FIG. 5, and sandwiched between a pair of upper and lower turn tables 71, 72 to which polishing pads 73, 74 are attached, and a polishing slurry is supplied and the upper and lower turn tables 71, 72 and the carrier 75 are rotated, and thereby the both surfaces of the wafers W are polished at the same time.
In the case of performing the double-polishing as described above, a peripheral portion (chamfered portion C) of the wafer W and an inside surface of the circular hole 78 in a carrier 75 are contacted and thereby scratch or impression in the chamfered portion C of the wafer W is generated. In order to remove the scratch and such, after the double-side polishing, it is general to polish the chamfered portion C of the wafer W.
Moreover, in a film-forming treatment step or a resist resin film-applying step in producing a device, occasionally, an oxide film or a nitride film is formed on the chamfered portion or a resist film adhered thereto. However, if the chamfered portion has surface roughness, these film components are in danger of being not removed in a subsequent cleaning step and so forth and remaining to be a dust generation source. However, when the chamfered portion is made to be a mirror surface, it becomes easy to remove an adhering resist film and such.
For polishing the chamfered portion, various types of polishing methods and polishing apparatus has been proposed. For example, as shown in FIG. 8(A), there is known a method in which the wafer W is held with a vacuum chuck stage 13 and the chamfered portion of an inclined wafer W is pressed with a certain pressure on a rotation dram 11 of which a polishing pad 12 is attached to the periphery, and thereby the chamfered portion is polished (see, International Publication WO 2002/005337).
Moreover, there are a method for polishing the entirety of the chamfered portion by, pressing each of polishing pads 21, 22 having inclined surfaces to be polished 21a, 22a and a polishing pad 24 having a perpendicular surface on the chamfered portion of the rotating wafer W as shown in FIG. 9, and a method for, polishing a chamfered surface of a main surface side of the chamfered portion of the wafer W with pressing a inverted cup-shaped polishing pad 31 on the chamfered portion, and then polishing the end surface (outermost peripheral surface) of the chamfered portion with pressing a perpendicular polishing pad 34 on the end surface, as shown in FIG. 10(A)(B).
However, when the chamfered portion is polished with inclining the wafer W with respect to the polishing pad 12, for example, as shown in FIG. 8(A) after the double-side polishing, intrusion of the polishing pad on the wafer surface (main surface) is caused and some region of a main surface in the vicinity of the chamfered portion is polished (in the present invention, referred to as “excessive polishing”), and flatness of the peripheral portion of the wafer made by double-side polishing becomes occasionally degraded. In particular, if a polishing pad made of flexible resin such as urethane is used, the excessively polished region is easily generated. Moreover, even in the case of polishing a chamfered portion as shown in FIG. 9 and FIG. 10, the similar excessively polished region is easily generated.
When the chamfered portion is polished after the double side polishing, there is a problem that even in any conventional method, excessive polishing that exceeds the border thereof with a main surface is easily caused and the excessive polishing has a harmful effect on a peripheral shape of the wafer main surface. In particular, as a region used in the wafer surface that is required in device production has been more expanded (the peripheral excluded region is more scaled down), influence of excessive polishing on the wafer has become larger.